On the basis of Rankine cycle, the water vapor power cycle has been widely used in the fields of nuclear power, combined fuel gas-steam cycle and coal-fired power station, etc. In these fields, the generation of water vapor with high temperature and high heat is the first step for the conversion from the thermal energy into the power. At present, there are mainly two types of equipments for the generation of water vapor, which are the natural cycle steam generator and the once-through steam generator. In comparison with the natural cycle steam generator, the once-through steam generator can directly generate overheated steam and steam with super high pressure and supercritical parameters, which has not only higher generating efficiency, but also a compact structure.
According to its way of arrangement in the once-through type steam generator, a hot water pipe can be classified into two types which are the straight pipe and the spiral pipe. In comparison with the arrangement of the spiral pipe, the structure of the once-through steam generator of a straight pipe type is simpler, but as the material of its heat exchanging pipe is different from that of its cylinder, there is a difference between their linear expansions, resulting in the concentration of stresses at the heat transmission pipe and the pipe plate, and affecting the safety of the operation of overall equipments. Although the total heat exchanging area of the once-through steam generator of spiral pipe type is relatively large, its structural feature can well solve the problem of stress concentration phenomenon, and it is more flexible in terms of space flexibility.
Because of the above advantages of the once-through steam generator of spiral pipe type, it is widely used in the fields of nuclear reactor electricity generation and power. The main designs are classified into two types which are the integrated design of large spiral pipe type and the separated modularization design.
The THTR-300 thorium high-temperature gas-cooled reactor in Germany, the Saint Flensburg high-temperature gas-cooled reactor in USA, the AGR type reactor in UK, and even the newest Sodium Cooled Fast Reactor all utilize the once-through steam generator of large spiral pipe type with multi-head winding and integration arrangement. One of the advantages of such steam generator is its compact structure. Furthermore, since the radius of curvature of the spiral is large, the volume inspection and surface inspection can be conducted. The main problems for such device include: 1) since the design can not be verified by conducting external thermal state test outside the reactor, the water flow side can not be reallocated in the operation, which is prone to result in the unevenness of steam temperature; 2) For the once-through steam generator of large spiral pipe type with integration arrangement, the spiral pipe in each layer needs independent tool pieces as the diameter of curvature of the spiral pipe in each layer is different, the processing expense thus is costly and the period is relatively long; 3) In order to prevent from the flow-induced vibration, more supporting plates are required, and the problem of local overstress for the heat exchanging pipes and the supporting plates is further highlighted.
The VG-400, ATY-50, ΓP-300 reactors in Russia and the 10 MW high-temperature gas-cooled test reactor in Tsinghua University all utilize separated modularization once-through steam generator. The main advantages for such type of steam generator are that the module can be produced in batches, the production cost is low, and each module can conduct external thermal state verification test outside the reactor. The main problems for such device include: 1) the structure is not compact enough; 2) the small radius of curvature of the spiral pipe can not conduct the volume and surface in-service inspection; 3) when a pipe blockage takes place, not only the water flow side is blocked, the side of high-temperature heat transfer material is blocked as well.